1. Field of the Invention
The present invention is directed to a process for producing hard plastic clad optical fiber with improved ultraviolet transmission. More specifically, it involves coating a core as it is being drawn with an active energy ray curable composition which is curable with energy below 800 keV and subsequently exposing the coated core to such radiation.
2. Prior Art Statement
In recent patents U.S. Pat. No. 4,511,209, 4,707,076 and 4,884,866 coatings and processes for achieving hard clad coatings for optical fibers have been described. In practice these hard coatings are valued for their ability to provide protection to the glass core while functioning as optical claddings. Being thin relative to the core enhances their core to clad fiber ratio and thus permits larger cores for medical applications which employ catheters of limited size or for bundle applications, medical or otherwise, where the efficiency of the bundle is increased by having more core area/bundle diameter. Being hard not only provides some protection for the glass core but also permits the use of crimp and cleave terminations as well as the pot and polish type, which may provide process or cost advantages.
As the use of optical fibers to carry laser energy in medical and in industrial applications has become accepted and valued, the trend for certain processes to be more efficient with higher frequency, i.e. shorter wavelength, radiation has raised a need for optical fibers with better transmission in the ultraviolet and near-ultraviolet regions of the electromagnetic spectrum. Heretofore the standard coatings/claddings described above were cured efficiently in-line on the fiber draw tower by the use of ultraviolet light with coatings/claddings containing ultraviolet initiators. The art for coating/curing protective coatings on optical fibers is described, for example, in U.S. Pat. Nos. 4,099,837 to Vazirani and 4,125,644 to A. D. Ketley et al.
The prior art initiators are highly ultraviolet absorbing species that either fracture or abstract a hydrogen atom from a neighboring molecule after absorbing the ultraviolet light, generating molecular radicals which then actively and very efficiently initiate polymerization and crosslinking in the bulk of the coating. Without the ultraviolet initiators, the coatings/claddings cure with great difficulty and certainly cannot cure fast enough within the bounds of the commercial fiber draw process to be useful compositions.
The presence of these ultraviolet initiators, which function by having strong ultraviolet absorptions, severely limits the transmission qualities of plastic clad optical fibers in the near ultraviolet and ultraviolet regions of the spectrum. In fact most current applications thus use glass-core/glass-clad type optical fibers for applications in these areas. Glass clad fibers are not the subject of the current patent, but suffice to note that there are certain advantages of plastic clad fibers that would make them desirable over the glass clad if their transmission in this region could be improved to near that of the glass clad fibers.
As noted in U.S. Pat. No. 4,884,866, compositions based on acrylates (methacrylates are to be understood as included in this term for the rest of this disclosure) can also be cured with other active energy rays such as electron beams or gamma rays. With these more energetic rays, the need for initiators may be reduced or eliminated. The problem with employing these rays is that such exposures of silica based optical fibers degrade the transmission of light throughout the spectrum--ultraviolet, visible and near infrared, as documented in a large and continually growing body of research on the effects of high energy ionizing radiation in optical fibers. (See e.g. D. L. Griscom, Phys. Rev. B40, 4224 (1989) and E. J. Friebele, et al J. Lightwave Technol. 6, 165 (1988).) Further, there are new studies directed to the detrimental effects of exposure of silica based optical fibers to intense ultraviolet lamps during the curing of the standard compositions. Results are inconclusive, but for a related silica glass used for containers and microscope slides, namely soda-lime glass, the darkening of the glass is quite similar for a given energy density deposition, regardless of whether by ultraviolet exposure or by electron beam exposure. Thus, prior art approaches to produce optical fibers, especially with the desirable hard plastic cladding here not easily yielded fibers with improved ultraviolet transmission, as are achieved by the present invention.